Methodologies
Electrical Resistivity Testing (ERT), Electrical Resistivity Imaging (ERI) or Tomography and Marine Electrical Resistivity Imaging (MERI): This simplified example is based on a Wenner array: Current (I) is induced between paired electrodes (C1, C2). The potential difference (ΔV) between paired voltmeter electrodes P1 and P2 is measured. Apparent resistivity (Δa) is then calculated (based on I, ΔV, electrode spacings). If the current electrode spacing is expanded about a central location, a resistivity–depth sounding can be generated. If the array is expanded and moved along the surface, 2-D or 3-D resistivity–depth models can be created.
Modified from “Introduction to Geotechnical Geophysics – N. Anderson and N. Croxton, Circular – Number E-C130, October 2008”.
Electrical Resistivity Testing (ERT) or Vertical Electrical Sounding (VES) consists of 4 electrodes firmly hammered into the ground and connected vie electrical cables to the main console. The electrodes and cables are moved at pre-determined spacings for each reading.
Electrical Resistivity Imaging (ERI) or Electrical Resistivity Tomography (ERT) consist of several electrodes (e.g., 64, 72, 84, 96 etc.) firmly hammered into the ground and connected via 4 electrical cables to the main console. Normally, 1 or 2 cables at the beginning of the layout and respective electrodes are moved for each reading, i.e. “roll-along”. This generates an overlap of data with each reading.
Marine Electrical Resistivity Imaging (MERI) consist of several electrodes towed from a vessel that are connected to the main console on board vessel.
Figure 1: Sample of Electrical Resistivity Testing (ERT) tabulated data (extract from ERT report)
Figure 2: Sample of Electrical Resistivity Imaging (ERI)
Figure 3: Sample of Marine Electrical Resistivity Imaging (MERI)